The present invention relates to a circuit arrangement and a method for the clocked actuation of a semiconductor switch connected in series with an inductive load, and in particular, a circuit configured to reduce electromagnetic radiated interference, which arises during switching processes.
FIG. 1 illustrates a prior art series circuit (connected between a supply potential V and reference-ground potential GND) comprising a semiconductor switch in the form of a power transistor T and comprising an inductive load having a freewheeling diode D connected in parallel with the load. With clocked actuation of the semiconductor switch, a current flows via the semiconductor switch T and the load L to reference-ground potential GND during the periods in which the semiconductor switch T is on. While the semiconductor switch T is subsequently off, the energy stored in the coil beforehand causes a current to flow from the coil via the freewheeling diode, as a result of which the coil turns off. If the periods for which the semiconductor switch T is off in this case are so short that the load L is not respectively able to turn off completely during these periods, then after the semiconductor switch T has turned on a current continues to flow via the diode D, said current decreasing as the length of time for which the semiconductor switch is on increases. In this case, electromagnetic radiated interference is particularly great at the time at which a flow of current ID through the diode ends and the total current through the load L is taken on by the semiconductor switch T. At this time, the arithmetic sign of the voltage UL present across the diode D and the coil changes from a value that is negative with respect to reference-ground potential GND to a positive value.
In this case, it holds that the electromagnetic radiated interference is more intense the greater the inductance of the load L and hence the change in current in the semiconductor switch T.
To reduce electromagnetic radiated interference, it is known practice to actuate the semiconductor switch T using a suitable actuation circuit (not shown in more detail) such that edges of the load current profile have a shallow gradient. A drawback in this context is that the semiconductor switch T is on for a correspondingly long time, which limits the maximum switching frequency for clocked actuation of the semiconductor switch.
To reduce electromagnetic radiated interference, attempts are therefore made to identify the time interval for the maximum change in current in the semiconductor switch in order to be able to flatten or round off the edges of the current profile during this period of time by means of suitable actuation of the semiconductor switch. This flattening of the edges is done, for example, by temporarily limiting the semiconductor switch somewhat using a suitable actuation circuit in order to increase the semiconductor switch's load path resistance and thereby to reduce the change in the load current through the semiconductor switch T. By way of example, the limiting is effected by temporarily reducing the gate charging current in a semiconductor switch in the form of a power MOSFET.
To detect the interval of maximum changes in current, it is possible to evaluate the voltage across the diode D or across the load L. FIG. 2 schematically illustrates the time profile for the voltage drop across the diode D, where ts denotes a time at which the initially off semiconductor switch T is turned on again. At first, this voltage UL remains at a negative value whose magnitude is dependent on the coil current, and it then rises quickly toward positive values, the total current being taken on by the semiconductor switch T from the time at which the voltage UL becomes positive.
To detect this rising edge, the voltage drop across the diode can be compared with a prescribed reference value which is between the forward voltage of the diode D and reference-ground potential GND, and the semiconductor switch T can be actuated such that the current edge is flattened as soon as the diode voltage UL reaches the reference voltage.
This practice has two drawbacks, however. First, after initially remaining approximately constant at a negative value, the voltage across the diode D rises very quickly to positive values. Usual periods of time for the transition between the negative voltage value with the greatest magnitude and positive values are between 100 ns and 300 ns. Detection of this steep edge using the reference value Uref presupposes a fast comparator circuit and also a fast actuation circuit in order to start rounding off the current edge through suitable actuation of the semiconductor switch T even before positive voltage values are reached and the semiconductor switch then takes on the total current. This can be achieved, by way of example, using circuits in bipolar technology, which can be implemented only with a greater areal involvement than CMOS circuits, however. In addition, fast comparator and actuation circuits can be implemented only with a high level of circuit complexity, which means additional areal involvement.
Second, the choice of switching threshold influences the electromagnetic interference and the maximum possible switching frequency of the semiconductor switch. If the switching threshold is chosen to be too high, for example just a little lower than reference-ground potential GND, then flattening of the switching edges starts too late and the electromagnetic radiated interference is barely reduced. If the switching threshold is too low, flattening of the switching edge starts too early and the period of time before the semiconductor switch turns on completely increases. In the example in FIG. 2, the threshold is below the minimum value which the diode voltage reaches for an assumed coil current of 1A, which means that the switching edges are always flattened on the basis of a comparison between the diode voltage and the reference voltage. In general, it holds that the smaller the impressed coil current on which the voltage drop across the diode is dependent, the earlier flattening of the current edge starts and the longer the turn-on process takes.